THE  UNIVERSITY 

OF  ILLINOIS 

LIBRARY 


AGRICULTURAL 
LIBRARY 


UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.   221 


SNAPDRAGON  RUST 


BY  GEOEGE  L.  PELTIER 


URBANA,  ILLINOIS,  AUGUST,  1919 


CONTENTS  OF  BULIJSTIN  No.  221 

PAGE 

INTRODUCTION 535 

HISTORY   AND  DISTRIBUTION 535 

SYMPTOMS  OF  TJIE  DISEASE 536 

DESCRIPTION  OF  THE  FUNGUS 538 

HOST  EELATIONSHIP  AND  RESISTANCE 538 

PREVENTION  AND  CONTROL  OF  SNAPDRAGON  BUST 

Experiments  with  Fungicides 541 

Effect  of  Cultural   Methods .544 

SUMMARY  548 

RECOMMENDATIONS   .  ..548 


SNAPDRAGON  RUST 

BY  GEOBGE  L.  PELTIEE,  ASSOCIATE  IN  FLORICULTURAL  PATHOLOGY 

INTRODUCTION 

The  snapdragon  (Antirrhinum  majus  Linn.)  has  been  grown  for 
many  years  as  a  garden  flower.  Within  the  last  six  or  seven  years 
it  has  also  become  one  of  the  more  important  minor  crops  in  com- 
mercial greenhouses.  As  a  result,  a  large  number  of  new  varieties 
suitable  for  growing  under  greenhouse  conditions  have  been  originated 
and  introduced  into  all  parts  of  the  country. 

•Coincident  with  this  more  intense  cultivation  under  glass,  a  num- 
ber of  fungous  diseases  have  appeared  on  the  plant.  One  of  these 
has  become  serious  within  the  past  four  years.  This  is  the  snapdragon 
rust  (Puccinia  antirrhini  D.  &  H.),  which  first  became  evident  in 
Illinois  in  1913  and  since  that  time  has  caused  much  loss  thruout 
the  state. 

HISTORY  AND  DISTRIBUTION 

Snapdragon  rust  was  reported  first  in  this  country  in  1903,  by 
Blasdale,1  who,  during  the  summer  of  1895,  had  found  it  destroying 
a  number  of  plants  growing  in  the  gardens  of  California.  He  says : 
"Since  that  year  the  fungus  has  appeared  every  season  in  which  an 
attempt  was  made  to  raise  this  plant,  and  in  every  case,  destroyed 
the  plants  shortly  after  they  had  reached  the  flowering  stage.  Fur- 
ther observations  have  shown  that  the  disease  is  a  common  one  in 
the  region  around  San  Francisco  Bay  tho  I  have  no  knowledge  of  its 
occurring  elsewhere  in  the  state."  Specimens  were  submitted  by 
Blasdale  to  Holway  and  Dietel,  and  a  description  of  the  species  was 
published  by  them  in  1899  under  the  name  of  Puccinia  antirrhini.2 

Before  1895,  however,  specimens  of  rust  on  snapdragon  had  been 
collected  a  number  of  times  in  California,  as  shown  by  a  letter  from 
Dr.  J.  C.  Arthur:  "In  my  herbarium  I  have  specimens  of  the  rust 
fungus  P,  antirrhini  from  the  following  localities ;  Santa  Cruz,  Cal., 
1879 ;  Berkeley,  Cal.,  1897 ;  Ukiah,  Cal.,  1902 ;  Whittier,  Cal.,  1909 ; 
and  Portland,  Ore.,  1909."  No  further  reports  of  this  rust  can  be 


NOTE. — The  experimental  work  of  this  bulletin  \vas  completed  in  June,  1916. 
Since  September  1,  1916,  Dr.  Peltier  has  been  Plant  Pathologist  at  the  Alabama 
Experiment  Station,  Auburn,  Ala. 

Blasdale,  W.  C.  On  a  Bust  of  the  Cultivated  Snapdragon.  Jour.  Mycol., 
9,  81-82.  1903. 

2Hedwigia,  36,  298.     1899. 

535 


536  BULLETIN  No.   221  [August, 

found  in  the  literature  up  to  the  outbreak  in  the  vicinity  of  Chicago 
in  1913.  It  is  therefore  thought  that  it  was  probably  localized  on 
the  west  coast  for  a  long  period  of  time. 

No  mention  of  the  occurrence  of  snapdragon  rust  in  Europe  can 
be  found  in  European  literature,  altho  this  rust  was  named  by  a 
German  mycologist  from  specimens  sent  to  him  from  the  United  States. 

During  the  month  of  July,  1913,  the  first  report  of  the  presence 
of  snapdragon  rust  on  field  plants  in  Illinois  was  sent  to  the  Illinois 
Experiment  Station  from  a  region  near  Chicago.  By  the  end  of  the 
following  month,  four  new  areas  of  infection  had  been  found  in  that 
vicinity.  In  January,  1914,  the  writer  visited  the  section  and  found 
rust  on  cuttings  and  mature  plants  in  a  number  of  greenhouses.  Be- 
tween that  time  and  the  fall  of  1914,  the  presence  of  snapdragon  rust 
was  reported  by  a  dozen  growers  from  various  parts  of  the  state. 
Soon  thereafter  inquiries  were  received  from  Indiana  and  Ohio,  and 
by  January,  1915,  rust  had  been  found  in  Wisconsin,  Iowa,  and 
Massachusetts.  Bailey,1  in  a  short  note,  described  this  disease  from 
Oregon  in  1915.  However,  Dr.  Arthur  had  had  specimens  from  that 
state  since  1909.  Later  in  the  year,  rust  specimens  were  received  from 
Michigan  thru  Dr.  Arthur.  Only  a  short  time  ago  (1916)  specimens 
were  collected  in  Alabama  by  the  writer.  At  the  present  writing, 
snapdragon  rust  is  found  practically  wherever  this  crop  is  grown  in 
the  greenhouse,  and  its  distribution  may  be  said  to  be  coextensive 
with  the  entire  area  of  the  United  States. 

The  rapid  spread  of  this  rust,  especially  in  the  Middle  West,  is 
probably  related  to  the  fact  that  young  plants  may  be  shipped  to 
all  points  from  infected  areas.  While  it  has  been  impossible  to  trace 
directly  the  origin  of  the  first  infection  in  Illinois,  it  is  thought  to 
have  been  introduced  on  cuttings  or  plants  from  California. 

SYMPTOMS  OF  THE  DISEASE 

Snapdragon  rust  attacks  seedlings,  cuttings,  and  mature  plants 
both  in  the  field  and  in  the  greenhouse.  It  is  most  severe  on  cuttings 
and  on  plants  just  before  blooming  time.  Leaves,  branches,  stems, 
and  occasionally  seed  pods  are  subject  to  attacks  of  this  rust.  The 
fungus  is  limited,  as  a  rule,  to  the  underside  of  the  leaves,  while  the 
branches  and  stems  are  attacked  at  any  point.  The  more  sheltered 
points  along  the  stem  are,  as  a  rule,  infected  first. 

From  eight  to  fourteen  days  after  a  plant  has  been  exposed  to  rust, 
light-colored,  slightly  raised  pustules  or  spots  appear.  Within  a  few 
days  these  break  out  into  dusty  brown  masses.  Occasionally  pustules 


JBailey,  F.  D.     Second  Bien.  Crop  Pest  and  Hort.  Kpt.,  Ore.  Agr.  Exp.  Sta. 
(1913-14)    p.    281.     1915. 


1919} 


SNAPDRAGON  BUST 


537 


are  produced  on  the  upper  surfaces  of  the  leaves.  Under  favorable 
conditions  the  pustules  increase  rapidly  in  number  on  both  the  leaves 
and  the  stems,  causing  the  death  of  the  plant  in  a  short  time  (Fig.  1). 


FIG.  1. — SNAPDRAGON  PLANT  KILLED  BY  RUST 

The  fungus  is  able  to  attack  plants  at  all  stages  of  growth.  -  Cut- 
tings are  extremely  susceptible,  while  seedlings  are  not  attacked  until 
several  pairs  of  leaves  are  formed.  The  appearance  of  the  rust  on 
cuttings  and  seedlings  is  the  same  as  described  above  for  the  mature 
plant. 

Rust  in  the  greenhouse  may  occur  thruout  the  year,  but  it  is  es- 
pecially prevalent  during  the  warmer  months,  decreasing  in  winter, 


538  BULLETIN  No.   221  [August, 

when  the  temperatures  are  low  and  uniform.  However,  if  the  tem- 
perature of  the  house  is  high  during  the  winter,  rust  will  be  as  severe 
as  in  the  summer.  In  the  field,  rust  generally  appears  in  Illinois 
during  the  latter  part  of  June  or  in  July  and,  in  severe  cases,  kills 
the  plants  either  before  or  at  the  time  of  flowering. 


The  life  history  of  the  fungus  is  not  completely  known,  but  for 
the  present  the  organism  may  be  classed  as  a  Hemi  form  of  rust. 
Only  the  uredinial  and  telial  stages  are  known.  The  uredinial  stage 
is  the  usual  method  of  propagation  both  in  the  field  and  in  the  green- 
house. The  spores  are  produced  in  pulverulent  sori,  which  are  gen- 
erally circular  (Fig.  2).  On  the  stems  and  branches  of  the  host,  the 
sori  may  become  elongate.  The  urediniospores  are  spherical  to  ellip- 
soidal, varying  from  22  to  30  by  21  to  25  microns.  They  are  yellow 
to  dark  brown  in  color,  and  are  borne  on  short  pedicels  which  are 
deciduous.  The  spore  walls  are  echinulate,  or  warty,  and  have  two 
and  rarely  three  germ  pores.  The  spores  germinate  readily  in  water 
or  on  gelatine  in  from  twenty-four  to  forty-eight  hours. 

The  telial  stage  does  not  occur  until  the  latter  part  of  the  season ; 
it  may  be  found  on  the  plants  both  in  the  field  and  in  the  greenhouse. 
The  teliospores  are  produced  either  in  the  uredinia  mixed  with  the 
urediniospores,  or  in  separate  sori  on  the  stems  or  branches  of  the 
plant  (Fig.  3).  The  telia  are  elongate  and  quite  distinct  from  the 
uredinia.  The  teliospores  are  thick-walled,  two-celled,  and  somewhat 
club-shaped.  They  are  a  deep  brown  in  color,  and  bear  a  persistent 
pedicel.  The  spore  measurements  vary  from  36  to  50  by  17  to  26 
microns. 

All  efforts  to  germinate  the  teliospores  have  failed.  The  spores 
were  subjected  by  the  author  to  alternate  wetting  and  drying,  to  high 
and  low  temperatures,  and  to  various  outdoor  conditions  thru  the 
winter  and  the  summer.  The  gelatine  plate  method  was  also  tried 
under  different  conditions,  but  with  no  success. 

The  mycelium  of  the  fungus  is  localized  and  does  not  spread  in 
the  tissues  of  the  plant  beyond  the  area  first  infected. 

HOST  EELATIONSHIP  AND  RESISTANCE 

In  a  study  of  the  Scrophulariaceae,  to  which  family  the  snap- 
dragon belongs,  it  is  found  that,  with  the  exception  of  several  native 
species,  all  the  genera  closely  related  to  Antirrhinum  have  been  in- 
troduced fronTEurope.  Since  specimens  of  this  rust  were  collected 
in  California  as  far  back  as  1879,  that  state  is  supposed  to  be  its 
native  habitat.  The  original  host  of  the  fungus  is  still  unknown. 


1919] 


SNAPDRAGON  BUST 


539 


FIG.  2. — LOWER  AND  UPPER  LEAF  SURFACES  OF  SNAPDRAGON  SHOWING  THE 
BUST  PUSTULES,  OR  SORI  (UREDINIA) 

Blasdale1  in  his  publication  makes  the  following  statements : 

"...  Thus  far  only  four  species  of  rust  inhabiting  scrophulariaceous  genera 
have  been  found  in  the  state.  . . . 

"The  two  species  of  Puccinia  (P.  wulfeniae  D.  &  H.  on  Wulfenia  cordata 
and  P.  rufescens  D.  &  H.  on  Pedicularis  semiborata')  present  decidedly  different 
characters  from  those  of  the  species  to  which  the  snapdragon  disease  is  due.  . . . 
The  first  mentioned  species  has  been  found  but  once  in  the  Bay  region  and  then 
only  in  small  amounts,  the  latter  is  known  only  from  the  Sierra  Nevada  moun- 
tains. . . . 

"Attempts  to  infect  various  other  species  of  scrophulariaceous  genera  with 
the  disease  have  been  successful  in  three  instances  only.  During  the  past  season 
it  was  found  that  the  plants  of  the  native  Antirrhinum  vagans,  grown  from  seed, 
were  attacked  with  nearly  the  same  degree  of  destructiveness  as  the  cultivated 
species.  Also  plants  of  Linaria  retiaulata  and  L.  amethystina  were  attacked  by 
the  disease  but  to  a  much  less  degree  than  the  other  species.  . . . 

"Presumably  then  the  snapdragon  rust  originated  on  the  wild  form  of  Antir- 
rhinum but  thus  far  the  disease  has  never  been  found  on  the  plants  growing 
spontaneously,  though  diligent  search  has  been  made  for  it  whenever  the  opportu- 
nity was  offered. 

' '  The  only  alternative  is  to  assume  that  it  is  a  case  of  adaption  of  a  species 
from  a  host  plant  belonging  to  a  different  order." 


*Loc.  cit. 


540 


BULLETIN  No.   221 


[August, 


A  number  of  cross-inoculations  with  several  species  of  Antirrhi- 
num, a  large  number  of  commercial  varieties  of  A.  majus  Linn.,  Lina- 
ria  vulgaris  Mill,  (the  common  butter-and-eggs),  and  a  number  of 
species  and  commercial  varieties  of  Linaria,  was  attempted  by  the 
author  for  two  seasons,  but  with  little  success.  No  rust  infection  was 
obtained  on  Linaria  vulgaris  or  on  any  other  species  or  commercial 
varieties  of  Linaria.  This  was  true  also  of  Antirrhinum  species,  with 
the  exception  of  A.  majus  and  its  numerous  varieties.  On  the  latter, 
infection  was  very  severe.  In  one  case  flaking  occurred  on  A.  mau- 
randioides  Gray,  a  climbing  plant  native  to  the  Southwest,  several 
urediniospores  being  produced.  All  the  inoculations  were  made  with 
urediniospores  under  the  most  favorable  conditions.  The  rust  of 
snapdragons  appears,  then,  to  be  closely  limited  to  Antirrhinum  majus 
and  its  varieties. 

These  results  substantiate,  in  part,  those  obtained  by  Blasdale, 
and  in  all  probability  the  native  host  of  this  rust  is  to  be  found  out- 
side of  the  Scrophulariaceae.  There  is  of  course  the  possibility  of 
the  rust  having  been  introduced  into  California  on  some  plant  from 
Asia. 

Finding  that  the  rust  was  apparently  confined  to  A.  majus  and  its 
varieties,  the  author  conducted  inoculation  tests  on  some  sixty  or  more 
varieties  in  the  greenhouse  and  in  the  field,  over  a  period  of  three 


FIG.  3. — ELONGATE  BUST  PUSTULES,  OR  SORI   (TELIA),  ON  STEMS  OF 
SNAPDRAGON 


1919}  SNAPDRAGON  BUST  541 

years.  These  varieties  represented  practically  all  the  different  types 
offered  by  the  American  seedsmen.  All  degrees  of  variations  were 
represented,  in  thickness  of  the  epidermis,  in  the  presence  of  a  cutin- 
ized  epidermis,  and  in  protective  appendages,  such  as  hairs,  etc. 
The  results  of  these  tests  indicated  that  all  varieties  were  equally 
susceptible  to  the  rust.  Infection  took  place  in  from  nine  to  sixteen 
days  after  inoculation. 

PREVENTION  AND  CONTROL  OF  SNAPDRAGON  RUST 

Unfortunately  little  or  no  progress  has  been  made  toward  the  de- 
velopment of  rust-resistant  varieties  of  snapdragon,  so  that  the  elimi- 
nation of  the  fungus  by  this  method  is,  at  the  present  time,  out  of 
the  question.  The  following  experiments  with  fungicides  also  show 
rather  conclusively  the  ineffectiveness  of  spraying  as  a  means  of 
controlling  the  rust.  Thru  the  adoption  of  certain  cultural  methods, 
however,  the  possibility  of  at  least  partial  control  in  the  greenhouse 
is  assured,  while  the  disease  can  be  avoided,  or  eliminated  in  time, 
by  the  propagation  of  plants  from  seed. 

EXPERIMENTS  WITH  FUNGICIDES 

During  the  first  week  in  June,  1915,  640  snapdragons,  repre- 
senting forty  varieties,  were  transplanted  from  four-inch  pots  into 
the  field.  About  two  weeks  were  allowed  to  elapse  before  the  first 
spraying  was  started.  By  this  time  the  plants  had  begun  to  put  out 
new  growth  and  were  fairly  well  established.  A  number  of  the  plants 
were  infected  at  the  time  of  transplanting,  and  by  the  end  of  the 
first  two  weeks  frequent  rains  had  brought  about  infection  on  prac- 
tically all  of  them.  The  plants  were  divided  into  four  equal  lots. 
One  set  was  sprayed  thoroly  with  Bordeaux  (4-4-50)  ;  a  second,  with 
ammoniacal  copper  carbonate ;  and  the  third  set,  with  Fungine  (1-30), 
a  prepared  fungicide  containing  potassium  sulfid.  The  fourth  lot 
served  as  a  check. 

Each  fungicide  was  applied  on  the  following  dates:  June  6  and 
26;  July  2,  9,  14,  and  20.  A  complete  record  of  every  plant  was 
made  after  the  last  application.  All  the  plants  had  become  badly 
rusted  and  many  were  completely  killed.  The  sprayed  plants  were 
in  as  bad  condition  as  those  in  the  check  plot.  Judging  from  the  re- 
sult of  this  experiment,  snapdragon  rust  once  it  has  made  its  appear- 
ance on  the  plants  cannot  be  controlled  in  the  field  by  the  fungicides 
used. 

In  the  fall  of  1915  spraying  experiments  were  started  in  each  of 
two  houses.  A  north  bench,  divided  into  ten  five-foot  sections,  was 
used  in  each  case.  Between  the  sections  a  double  cheesecloth  curtain, 
fastened  on  frames  four  feet  high,  was  set  up  to  eliminate  as  much 
as  possible  the  spread  of  rust  from  section  to  section.  Each  section 


542 


BULLETIN  No.   221 


[August, 


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was  planted  to  twenty  snapdragons  representing 
five  varieties;  namely,  Phelp's  White,  Mixed 
(seed  obtained  from  plants  killed  by  rust  in  the 
field  during  the  summer),  Giant  Lilac,  Buxton's 
Pink,  and  Nelrose.  Four  plants  of  each  variety 
were  set  in  a  row  across  the  bench,  twelve  inches 
being  allowed  between  the  plants.  These  plants 
had  been  grown  from  seed  in  the  usual  manner. 
When  benched  on  November  17,  1915,  they  were 
healthy  and  free  from  rust.  Unless  otherwise 
stated,  all  sections,  received  the  same  treatment 
as  is  used  in  growing  snapdragons  commercially. 

Beginning  on  November  20  each  section  was 
treated  as  shown  in  Fig.  4.  Where  the  combined 
treatment  was  used,  Bordeaux  was  applied  un- 
til the  spikes  developed  and  then  ammoniacal 
copper  carbonate  was  substituted  for  the  re- 
•  mainder  of  the  season.  The  latter  was  used  to 
avoid  discoloring  the  flowers  with  the  Bordeaux. 

For  several  weeks  the  sections  in  House  No.  1 
marked  "inoculated"  were  artificially  infected 
with  rust  spores  for  two  or  three  days  after  each 
spraying.  In  House  No.  2  natural  infection  was 
depended  on,  that  house  having  been  used  the 
previous  season  in  the  rust  work. 

In  House  No.  1  no  infection  occurred  in  any 
of  the  sections  until  January  29,  1916,  ten  weeks 
after  the  plants  were  first  inoculated.  The  sec- 
tion in  which  the  plants  were  watered  overhead 
(syringed)  and  inoculated,  was  the  first  section 
to  develop  signs  of  the  rust.  This  section  was 
followed  by  the  one  in  which  the  plants  were 
watered  from  overhead  and  not  inoculated.  By 
February  26  rust  was  present  in  all  the  sections 
but  one.  The  last  section  to  become  infected  was 
the  one  in'  which  the  water  was  applied  to  the 
soil  only.  Rust  was  first  observed  in  that  sec- 
tion on  March  25,  four  months  after  the  experi- 
ment was  started. 

The  results  in  House  No.  2  were  somewhat 
similar.  Rust  first  appeared  on  February  5  on 
the  plants  in  the  two  sections  watered  over- 
head, followed  on  February  19  by  the  sections 
watered  from  below,  the  sections  sprayed  every 
two  weeks  with  Bordeaux  and  ammoniacal 


1919] 


SNAPDRAGON  BUST 


543 


copper  carbonate,  and  the  sections  where  the  combined  treatments 
were  applied  every  week  and  at  two-week  intervals.  By  March  25 
all  the  plants  in  the  remaining  sections  were  rusted.  Apparently  the 
rust  appeared  later  under  natural  infection  than  under  artificial  in- 
fection; also  some  of  the  treatments  were  more  effective  in  the  sec- 
tions naturally  infected. 

A  final  examination  of  each  plant  was  made  during  the  latter 
part  of  May.  In  making  these  examinations,  the  plant  showing  the 
largest  amount  of  leaf  rust  was  used  as  a  standard  (100  percent)  by 
which  to  measure  the  amount  of  leaf  infection  on  all  other  plants. 
The  same  method  was  used  for  stem  infection.  After  the  percentage 
of  leaf  and  stem  infection  was  determined  for  each  plant,  the  results 
were  tabulated  from  the  standpoint  of  both  the  treatments  and  the 
varieties.  No  striking  differences  were  shown  in  the  susceptibility  of 
any  of  the  varieties.  In  the  following  table  is  given  the  average  per- 
centage of  leaf  and  stem  infection  for  each  section  in  each  house.  In 
comparing  the  results,  it  should  be  remembered  that  in  House  No.  2 
natural  infection  was  depended  on. 


EESULTS  OF  SPRAYING  EXPERIMENT  FOR  SNAPDRAGON  BUST,  IN  GREENHOUSE: 

1915-16 


Sec- 
tion 

Treatment 

Average  percentage  of 
infection 

House 
No.  1 

House 
No.  2 

Leaf 

Stem 

Leaf 

Stem 

1 
2 
3 

4 

5 
6 

7 
8 

9 
10 

Bordeaux  every  week  

55 
66 
80 

78 

.18 
62 
53 

48 

46 
56 

15 
15 
42 
33 

18 
26 
25 
6 

1 
14 

45 
47 
57 
36 

13 
41 
31 
57 

42 
68 

0 
0 
27 
4 

0 
1 
0 
17 

0 
16 

Ammoniacal  copper  carbonate  every  week  

Water  overhead  

Water  overhead  —  check  

Bordeaux  every  two  weeks  

Ammoniacal  copper  carbonate  every  two  weeks.  . 
Water  below  

Water  below  —  check  

Bordeaux   and    ammoniacal   copper    carbonate 
every  week   

Bordeaux   and    ammoniacal   copper    carbonate 
every  two  weeks  

The  results  in  House  No.  1  show  that  all  the  spraying  treatments 
were  ineffective  in  the  control  of  the  rust.  The  largest  amount  of 
infection  naturally  occurred  in  those  sections  in  which  the  plants  were 
watered  from  overhead  (syringed).  Where  only  the  soil  in  the  sec- 
tions was  watered  the  percentage  of  leaf  infection  was  lower  than 
in  any  of  the  sprayed  sections  except  one.  In  other  words,  this  treat- 
ment was  just  as  effective  in  the  control  of  rust  as  were  Bordeaux  and 
ammoniacal  copper  carbonate  when  applied  every  week. 


544  BULLETIN  No.   221  [August, 

In  House  No.  2  practically  the  same  results  were  obtained.  On 
the  whole,  however,  the  infection  in  this  house  was  not  so  great  as 
in  House  No.  1,  nor  was  it  so  uniform  owing  to  the  fact  that  natural 
infection  was  depended  upon.  In  the  last  three  sections  infection 
was  more  severe  than  in  the  other  sections  of  the  house  because  rust 
experiments  had  been  carried  on  in  the  bench  adjacent  to  these  dur- 
ing the  previous  season.  However,  the  plants  in  all  the  sprayed  sec- 
tions had  an  abundance  of  rust.  As  in  House  No.  1  the  greatest 
amount  was  found  on  the  plants  in  the  sections  watered  from  over- 
head. Decidedly  less  stem  rust  occurred  in  House  No.  2  than  in 
House  No.  1. 

From  the  results  of  these  experiments,  then,  it  may  be  concluded 
that  the  fungicides  used  will  neither  prevent,  check,  nor  control  snap- 
dragon rust  in  the  greenhouse;  it  can  be  further  concluded  that  by 
watering  the  soil  only,  in  the  bench,  and  avoiding  all  syringing,  the 
disease  can  be  held  in  check  better  than  by  the  use  of  fungicides. 

EFFECT  OF  CULTURAL  METHODS 

In  making*  a  study  of  the  fungous  diseases  of  plants,  a  study  of 
the  host  plant  and  the  methods  used  in  its  culture  is  as  necessary  as 
is  a  knowledge  of  the  fungus  causing  the  disease.  In  many  cases,  by 
modifying  the  cultural  methods  in  use,  attacks  of  fungi  can  be  to 
some  extent  avoided  or  prevented.  This  is  all  the  more  important  in 
the  case  of  the  snapdragon,  which  in  the  open  is  a  half-hardy  perennial. 
In  the  greenhouse,  the  external  conditions  under  which  the  plants 
are  growing,  such  as  temperature  and  moisture,  may  affect  the  plants 
in  such  a  way  as  to  make  them  more  susceptible  to  disease.  The  stems 
of  snapdragon  plants  in  the  greenhouse  are,  for  example,  much  more 
succulent  than  those  grown  outside  and  they  do  not  harden  as  in  the 
open.  A  succulent  stem  is  more  susceptible  to  snapdragon  rust  than 
a  hard,  firm  stem. 

Cuttings  from  Infected  Plants. — Cuttings  are  extremely  suscep- 
tible to  rust,  both  in  the  cutting  bench  and  afiter  they  are  potted.  The 
author  has  seen  three  thousand  cuttings  infected  with  rust  and  killed 
in  three  weeks  time.  He  has  also  seen  a  large  number  of  young  plants 
lost  just  after  being  potted.  Great  care,  therefore,  must  be  exercised 
to  prevent  infection  of  cuttings.  Such  infection  may  be  initiated  in 
the  cutting  bench  either  by  the  introduction  of  spores  or  by  the  use 
of  cuttings  made  from  infected  plants.  A  single  diseased  cutting 
may  mean  the  loss  of  nearly  all  the  cuttings  later  on.  It  is  extremely 
difficult  to  select  cuttings  free  from  the  disease  in  a  house  or  field 
where  the  rust  has  been  prevalent.  Certain  plants  may  appear  to  be 
free  from  rust,  but  cuttings  from  these  plants  may  lead  to  disappoint- 
ment, for  the  pustules,  which  give  the  first  appearance  of  the  disease, 


1919] 


SNAPDRAGON  BUST 


545 


do  not  become  evident  until  some  time  after  infection  has  taken  place. 
Cuttings  from  such  plants  carrying  the  disease  in  the  incipient  stage 
may  thus  become  centers  of  infection  for  an  entire  bench  of  cuttings. 

Syringing. — Two  important  conditions  favorable  to  the  develop- 
ment and  dissemination  of  the  rust  in  a  bench  of  cuttings  are  shade 
and  syringing.  For  a  week  or  ten  days  after  the  cuttings  are  placed 
in  the  sand  the  usual  procedure  is  to  syringe  the  cuttings  frequently 
and  keep  them  under  shade  in  bright  weather.  Such  conditions  are 
most  favorable  for  the  germination  of  the  rust  spores  in  that  the  nec- 
essary degree  of  moisture  is  maintained.  The  fungus  will  mature 
rapidly,  new  spores  will  be  produced,  and  soon  the  entire  lot  of 
cuttings  will  show  infection. 

Syringing  is  practiced  also  for  the  control  of  small  insects,  such 
as  the  red  spider.  When  syringing  is  practiced  in  an  infected  house, 
the  development  and  spread  of  the  rust  is  very  rapid.  To  illustrate 
this  point  the  following  experiment  was  carried  out  during  the  sea- 
son of  1914-15. 

Two  sets  of  twenty-three  healthy  snapdragon  plants  growing  in 
four-inch  pots  were  arranged  eight  feet  apart  on  a  greenhouse  bench. 
Two  badly  rusted  plants  were  set  in  each  section  in  the  positions  shown 
by  the  circles  in  Fig.  5.  In  the  first  set  only  the  soil  in  the  pots  was 
watered.  Care  was  taken  not  to  wet  or  disturb  the  foliage.  In  the 
second  section  the  soil  in  the  pots  was  watered  as  in  the  first  section, 
but  in  addition  the  plants  were  given  a  good  syringing  with  a  strong 
water  pressure.  The  syringing  was  done  from  one  side  of  the  bench, 
the  direction  of  the  current  being  indicated  by  the  arrow  in  Fig.  5. 


X  X  X  X  X 

x       x        •        O       • 

X          X          X  X          X. 


SECTION    1 WATERED    UNDERNEATH 


SECTION    2 SYRINGED 


FIG.  5. — EFFECT  OF  UNDERNEATH  AND  OVERHEAD  WATERING  (SYRINGING)  ON 
DEVELOPMENT  AND  SPREAD  OF  SNAPDRAGON  BUST;  1914-15 

The  open  circle  shows  the  location  of  the  infected  plants  placed  in  the  bench. 
The  arrow  indicates  the  direction  of  watering.  The  cross  shows  the  plants  to 
which  infection  was  carried. 


546  BULLETIN  No.  221  [August, 

The  experiment  was  discontinued  at  the  end  of  sixty  days  and  the 
plants  carefully  examined  for  rust.  Each  plant  infected  with  rust 
is  represented  by  a  cross.  Where  water  was  applied  to  the  soil  in 
the  pots  without  syringing,  no  infection  was  found.  Where  syringing 
was  practiced,  sixteen  plants,  or  69.5  percent,  were  badly  infected 
with  rust.  Note  that  the  infected  plants  are  in  the  position  where 
the  rust  spores  were  carried  by  the  water  from  the  infected  plants. 

Syringing,  then,  not  only  favors  the  development  of  the  rust,  but 
it  actually  carries  and  spreads  the  spores  to  other  plants.  Where 
no  syringing  is  done,  the  rust  spores  are  less  likely  to  germinate,  and 
consequently  less  infection  results. 

Under  the  spraying  experiments  reported  on  pages  541-44,  the  sec- 
tions in  which  the  soil  only  was  watered,  with  no  syringing,  were  the 
last  to  become  infected  and  showed  the  smallest  percentage  of  in- 
fection. In  those  sections  where  overhead  watering  was  practiced, 
the  plants  were  the  first  to  become  infected  and  showed  the  highest 
percentage  of  infection. 

Form  of  Plant. — The  form  of  the  plant  is  an  important  factor  in 
the  destructiveness  of  the  rust.  The  ideal  plant  should  fork  low  and 
not  have  more  than  five  or  six  stems.  Such  a  plant  is  less  liable  to 
destruction  by  the  rust  than  a  bushy  plant  with  dense  foliage.  This 
of  course  is  true  where  syringing  is  practiced.  The  reason  is  obvious ; 
the  plant  with  few  stems  growing  out  from  the  center  will  dry  off 
very  quickly  after  syringing,  while  several  days  may  be  necessary 
for  the  center  of  a  bushy  plant  to  dry  out.  As  the  rust  spores  re- 
quire from  twenty-four  to  forty-eight  hours  for  germination,  the 
evaporation  of  the  water  during  the  day  will  kill  the  germinating 
spores  and  so  prevent  infection.  Where  the  water  does  not  evaporate 
for  several  days,  rust  will  invariably  develop.  The  habit  of  the  plant, 
then,  is  of  some  importance  in  checking  the  spread  of  rust  in  the 
greenhouse. 

Destroying  Infected  Leaves. — An  attempt  to  control  the  rust  by 
pinching  off  and  destroying  the  infected  leaves  was  practically  a 
total  failure,  altho  a  partial  success  was  obtained  during  the  winter 
months  when  the  development  and  spread  of  rust  was  slow. 

Seed. — Experiments  were  also  conducted  in  an  effort  to  determine 
whether  rust  can  be  carried  on  the  seed.  Seed  representing  thirty- 
six  commercial  varieties  was  obtained  from  various  sources.  From 
this  seed  about  eighteen  hundred  plants  were  grown  to  maturity 
without  a  single  pustule  of  rust  developing.  Again,  seed  gathered 
from  badly  infected  plants  in  the  field  and  in  the  greenhouse,  sown 
and  grown  in  the  usual  manner,  developed  no  rust  whatsoever,  even 
tho  seed  was  taken  from  pods  on  which  rust  pustules  were  present. 


1919}  SNAPDRAGON  BUST  547 

It  can  be  safely  stated  that  rust  is  not  carried  on  seed  nor  propa- 
gated by  seed.  Altho  seeds  of  the  snapdragon  do  not  carry  the  rust 
from  season  to  season,  and  seedlings  are  not  easily  susceptible  to  in- 
fection, propagation  by  seeds  has  a  disadvantage  in  the  fact  that 
few  strains  are  fixed,  and  selection  must  be  practiced  in  order  to 
have  desirable  strains.  However,  the  use  of  seedlings  makes  it  pos- 
sible to  eliminate  the  rust.  If  in  a  rust-infected  house  or  field  all 
infected  plants  are  burned,  and  after  a  lapse  of  a  year  or  two  a  new 
beginning  with  seedlings  is  made,  clean,  healthy  plants,  will  result. 
From  such  plants  good  strains  can  then  be  developed  by  means  of 
careful  selection. 

Rust  in  the  Field. — The  florist  plants  but  few  snapdragons  in  the 
field,  altho  he  may  use  young  plants  in  the  spring  for  bedding  and 
border  work.  In  fact,  practically  all  snapdragons  grown  outside  are 
used  in  this  way.  Most  of  the  plants  are  grown  from  seed  and  planted 
out  the  latter  part  of  May. 

Rust  in  the  field  may  be  traced  to  two  sources — rusted  plants  of 
the  previous  season  and  the  use  of  infected  plants.  Experiments  have 
shown  that  the  urediniospores  can  winter  over  in  the  vicinity  of 
Urbana,  Illinois.  In  the  planning  of  the  beds  and  borders,  especially 
on  the  large  estates  where  the  greater  number  of  plants  are  used, 
changes  are  made  each  season  and  often  snapdragons  are  omitted 
from  the  scheme  of  planting.  So  few  of  these  plants  are  used  in 
decorative  and  landscape  work  that  the  spread  of  rust  from  one  plant- 
ing to  another  is  almost  negligible.  In  the  majority  of  cases  rust  is 
propagated  from  season  to  season  in  the  greenhouse  and  is  carried 
from  there  to  the  field.  During  favorable  seasons  in  the  field,  rust 
will  completely  kill  the  plants  just  before  blooming  time  and  by  fall 
the  whole  planting  will  be  dead.  It  is  very  important,  then,  to  use 
only  clean  plants  in  the  field;  which  means  that  the  grower  must 
eliminate  entirely  the  rust  from  the  greenhouse  before  he  can  be 
sure  that  he  is  using  clean  stock  in  the  field. 

A  few  growers  raise  snapdragons  thruout  the  year,  but  the  ma- 
jority prefer  to  plant  in  August  for  an  early  winter  crop  or  else 
reserve  them  to  follow  the  early  or  midseason  chrysanthemums.  Plants 
are  used  from  both  seeds  and  cuttings.  If  plants  are  rusted  when 
benched,  they  rarely  reach  maturity  or  produce  salable  spikes.  When 
clean  plants  are  benched,  little  or  no  rust  develops  until  in  the  early 
spring,  when  it  spreads  very  rapidly.  Temperature  is  of  importance 
in  the  development  of  rust;  it  has  been  observed  that  the  spread  of 
rust  is  checked  in  the  house  by  a  uniformly  low  temperature.  High 
temperatures  are  very  favorable  to  the  rapid  spread  of  the  fungus. 


548  BULLETIN  No.   221  [August, 

SUMMARY 

1.  Snapdragon  rust,  which  has  been  known  since  1879  in  Cali- 
fornia, was  introduced  into  Illinois  about  1913,  and  in  a  short  time 
spread  rapidly  thruout  the  United  States.     The  rapid  spread  of  the 
rust  was  probably  due  to  the  shipment  of  young  plants  from  infected 
areas. 

2.  The  disease  attacks  plants  at  all  stages  of  growth — from  cut- 
tings and  seedlings  to  mature  plants.    It  becomes  evident  on  leaves, 
branches,  stems,  and  occasionally  on  seed  pods,  both  in  the  field  and 
in  the  greenhouse. 

3.  The  cause  of  the  disease  is  a  fungus  known  as  Puccinia  an- 
tirrhini  D.  &  H.     Infection  occurs,  so  far  as  is  known,  only  thru 
the  urediniospores. 

4.  From  the  results  of  the  inoculation  experiments,   the  rust 
appears  to  be  limited  to  Antirrhinum  ma  jus  Linn,  and  its  varieties. 
All  varieties  of  this  species  are  equally  susceptible. 

5.  The  use  of  Bordeaux,  ammoniacal  copper  carbonate,  or  Fun- 
gine  can  neither  prevent,  check,  nor  control  snapdragon  rust  in  the 
field  or  in  the  greenhouse. 

6.  The  use  of  snapdragon  cuttings  or  seedlings  where  rust  is 
present  may  mean  a  complete  failure  of  the  crop. 

7.  Syringing  is  an  important  factor  in  the  prevalence  of  the 
rust  in  the  greenhouse.    It  is  a  means  of  dissemination  of  the  spores, 
and  it  produces  conditions  favorable  .for  the  germination  of  the  spores 
and  the  rapid  infection  of  the  plants. 

8.  Watering  the  plants  from  below  only,  so  as  to  avoid  wetting 
the  foliage,  is  a  means  of  checking  the  rust. 

9.  Snapdragon  rust  is  not  carried  on  the  seeds.    The  disease  can 
therefore  be  avoided,  or  eliminated  in  time,  by  the  propagation  of 
plants  from  seed. 

EE  COMMENDATIONS 

Snapdragon  rust  may  be  partially  controlled  in  the  greenhouse 
by  giving  attention  to  cultural  methods.  Growing  the  plants  under 
the  best  conditions,  in  a  clean,  well-kept,  and  well-ventilated  house 
will  check  to  some  extent  the  dissemination  of  the  disease.  Plants 
should  not  be  syringed  if  this  can  possibly  be  avoided,  but  instead 
the  soil  only  should  receive  water  when  the  plants  require  it. 

In  order  to  eliminate  the  rust,  it  is  recommended  that  all  infected 
material  be  destroyed,  the  house  cleansed,  and  after  a  year  or  two 
new  stock  secured  which  is  free  from  rust.  The  latter  may  be  secured 
by  the  use  of  seed  and  the  practice  of  selection. 


UNIVERSITY  OF  ILLINOIS-URBAN* 


